1. Field of the Invention
The present invention relates to an apparatus and method for kneading rubber or a rubber composition. The apparatus and method are used for masticating rubber to improve flowability/processability of the rubber and for kneading a material including rubber and various compound agents.
2. Description of the Related Art
In producing a kneaded material of a rubber composition including rubber and various compound agents, in particular, in producing a tire rubber compound primarily made up of natural rubber, a batch method for continuously kneading a predetermined amount of raw material once is often used. In this case, the batch-type kneading operation is repeated until formability in, for example, an extrusion process and dispersibility of compound agents reach desired levels, that is, until a desired viscosity level and a dispersibility level are reached. This kneading process is usually called a re-mill process, in which a batch-type kneading apparatus, such as an internal mixer, is used.
However, when kneading is carried out using such a batch-type kneading apparatus, after the kneading step, the material needs to be transported from the kneading apparatus to an extruder using, for example, a transporter in order to form the material into a predetermined shape, so that a step for transporting a lump of kneaded material is required. Therefore, the kneading apparatus tends to become more sophisticated and larger, and manual operation of a worker needs to be relied on, so that labor cannot be saved. In kneading using a batch-type kneading apparatus, in order to prevent deterioration of the material and to increase kneading efficiency, it is necessary to prevent the temperature from becoming too high during kneading, so that, ordinarily, a once-kneaded lump of material is formed in a sheet and cooled in order to re-mill the material again. The re-mill step is carried out until the viscosity is reduced to a desired viscosity. The re-mill step is often carried out up to approximately five times. Therefore, this is not desirable from the viewpoint of increasing productivity.
A kneading apparatus of a rubber composition for continuously kneading the material from the viewpoint of increasing productivity and the like is disclosed in Japanese Unexamined Patent Application Publication No. 11-262945. In the kneading apparatus, a double screw extruder, which is a typical example of a kneading extruder of a plastic composition, is used to knead rubber. An opening for putting in rubber is provided in the double screw extruder, and an extruder for supplying rubber is connected to the opening. While continuously supplying a rubber composition, the rubber composition is continuously kneaded by the double screw kneading extruder.
Therefore, in the continuous kneading apparatus disclosed in the aforementioned document, adjustment of the kneading state, that is the viscosity level and the level of dispersibility of compound agents is limited to a range which can be achieved by rotation of the double screw extruder, so that it is difficult to adjust the viscosity and the dispersibility of, for example, a high-viscosity rubber composition like that adjusted by the batch-type kneading apparatus. During kneading with the double screw extruder, the kneading state changes when, for example, the temperature increases or shearing is accelerated as the kneading operation progresses. It is, as a matter of course, difficult to properly set the kneading conditions in accordance with changes in the state of the material which is being kneaded.